Merge pull request #15206 from grafana/storybook/valuemappingseditor

Adding ValueMappingsEditor to Storybook

packages/grafana-ui/src/components/ValueMappingsEditor/ValueMappingsEditor.story.tsx

+import React from 'react';
+import { storiesOf } from '@storybook/react';
+import { action } from '@storybook/addon-actions';
+import { ValueMappingsEditor } from './ValueMappingsEditor';
+
+const ValueMappingsEditorStories = storiesOf('UI/ValueMappingsEditor', module);
+
+ValueMappingsEditorStories.add('default', () => {
+  return <ValueMappingsEditor valueMappings={[]} onChange={action('Mapping changed')} />;
+});